Obama: Boston Explosions Investigated as Terrorism

President Obama urges Americans to `speak up' to authorities if they see something suspicious.

WASHINGTON (AP) -- President Barack Obama says the explosions at the Boston Marathon are being investigated as an act of terror, although authorities still don't know who is responsible.

He called the bombing "a heinous and cowardly act" used to target innocent civilians. Obama spoke to reporters at the White House after a briefing by his national security team.

President Barack Obama has ordered flags at the White House and all government buildings to be flown at half-staff in honor of the Boston Marathon explosion victims.

Obama signed a proclamation making the order Tuesday morning, calling it a "mark of respect for the victims of the senseless acts of violence" perpetrated in Boston the previous day. He ordered flags to remain lowered through sunset Saturday.

Three people were killed, including an 8-year-old boy, and more than 170 were wounded in Monday's bombing at the famous marathon's finish line.

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